Studies have been extended on the role of fibronectin in phagocytosis by monocytes. Anti-fibronectin monoclonals have been used to purify fibronectin fragments. The ability of these fragments to 1) bind to monocytes and 2) enhance phagocytosis via the C3b receptor have been examined. We have shown that fibronectin fragments which bind well to monocytes are not sufficient to stimulate phagocytosis, and that another molecular fragment is required. We have localized that second fragment outside of a 70 kd fragment which contains the cell binding region of fibronectin. We have some evidence that this second signal comes from a region of the molecule around the gelatin binding domain and the heparin I domain. We have initiated studies on the effects of laminin on monocyte phagocytosis. Mouse laminin increases ingestion of EC4b through a mechanism similar to the fibronectin effect. Anti laminin antibodies inhibit this phagocytosis which is dependent on laminin concentration and the amount of C4b on the opsonized erythrocytes. Laminin has some effect in the fluid phase, as well as when plated onto surfaces. The laminin receptor of cultured human monocytes is an approximately 80,000 dalton glycoprotein with less than 5,000 copies present on the cell surface.